The invention can be implemented to advantage in a variety of sound pick up applications requiring sensing of a specific source within a background of other sound sources. The invention makes use of a directional optical beam to illuminate the acoustic source of interest and pick up its sound-related vibrations, thereby identifying the source of interest. As the acoustic impedance of essentially all sources of interest, ranging from the human body, through musical instruments to artificial sound generators, is several order of magnitude larger than that of air, airborne sound from other sources is almost entirely reflected by the surfaces of the source of interest so that the optical beam picks up only the sound generated by the source of interest. To overcome instances where the optical detection of the surface vibrations of the sound source generate a different acoustic signature from that normally generated into air, an ordinary microphone is added. By fusing the signals from the optical pick-up and the airborne detection with the microphone, the normal airborne sound quality is accomplished, but background sound is significantly suppressed.
The ability to pick up voice signals from a distant person is of special interest here. In this case the optical sensor must pick up the minute vibrations generated in the speaker's body during speech. The optical detection of such vibrations, with typically sub-nanometer amplitudes, on a human skin with multi-micrometer roughness is a challenge addressed in this invention. The challenge is further exacerbated by the necessity to track and alleviate the effects of the relative motion of the target. Even a person at rest is expected to move randomly on a millimeter scale. In other words the optical detection scheme must be capable of detection of sub-micrometer vibrations on a rough surface that moves randomly. The surface roughness generates speckles which degrade the sensitivity of standard detection schemes, the lateral relative motion of the target introduces variations in the speckle patterns and the axial relative motion of the target introduces variation in “work-point” of interferometric setups. The present invention alleviates these practical difficulties.